The fatty acid composition of the alga Chlorella saccharophila was investigated under different growth conditions. Using glucose as the sole carbon source, heterotrophically-grown Chlorella saccharophila produced a greater proportion of the polyunsaturated fatty acids (C18: 2 and C18: 3) than photosynthetic cultures, with linoleic acid (C18: 2) predominating. An unexpected discovery was the observation that at the lowest glucose concentration (2.5 gl-1) the lipid content of the algae increased to between 36-47% of the cell weight, depending on the temperature. At glucose concentrations of 5 g l-1 or more, the lipid content fell to 10-12% of the cell, although total fatty acid yield was higher due to higher biomass concentrations. Aeration of heterotrophic cultures promoted the production of unsaturated fatty acids compared to non-aerated cultures.